A gated community … no place for Dana and Gren.
Photograph: Radharc Images/Alamy

But what does the monster want? It’s a modern question to ask of an ancient story. Maria Dahvana Headley’s muscular, bloodthirsty novel is a contemporary retelling of Beowulf from the perspective of Grendel’s mother: nameless in the poem, here she’s called Dana Mills, and her son is Gren. In the Old English original, Grendel is simple malignancy, a grotesque “other” who stalks Heorot Hall and savages its inhabitants; when Beowulf kills him, his mother returns for vengeance. Over 800 years later, Mary Shelley’s Frankenstein gave us a monster with not just pathos but psychology; Headley’s story owes as much to the Romantic era as the Anglo-Saxon.

Dana is a US marine, and her military background makes her a convincing threat as well as enabling a sharp comment on literary traditions of sexism. A list of “selected translations” at the start of the novel gives Headley’s definitions of three words: the masculine noun aglæca means “fighter, warrior, hero”; the feminine counterpart aglæca-wif means “wretch, monster, hell-bride, hag”. What is heroism in a man becomes horrifying in a woman. (The third word is the indispensable, untranslatable Old English call to attention hwæt: Headley gives a whole list of possibilities, which later turn up as section headings.)

Kidnapped from a desert battlefield and apparently beheaded on camera, Dana somehow survives – less one eye, but with PTSD, amnesia and an unexplained pregnancy. “Rape? Or consensual?” demands an interrogator after she’s picked up by her comrades. It’s a difficult question, not just because of memory loss: “One answer means I’m a victim, and the other means I’m a collaborator, and I don’t know, so I don’t answer.” Shipped back home, she slips out of custody, and makes her way to the mountain near the town where she grew up.

That town no longer exists. Where it stood, there is now a gated community of mini-mansions named Herot, and the grandest of these is Herot Hall, occupied by the founder’s son Roger Herot, his wife Willa and their son Dylan. While Dana’s sections are first-person, the Herot chapters are told in the third person and centred on Willa, with waspish humour at the expense of her Waspy life. (Willa, we learn, has only feared once that her husband might betray her: a time when he “began to pour himself scotch and sob at the dining table, using words like existential and guitar”.)

Herot has no place for Dana and Gren, her miraculous child, born fatherless and with teeth. How she loves him, how she fears for him: “His eyes are black gold. He’s all bones and angles. He’s almost as tall as I am and he’s only seven. To me, he looks like my son. To everyone else? I don’t know. A wonder? A danger? A boy with brown skin?” The echo here is not centuries old, but six years: Trayvon Martin, the unarmed black teenager shot dead in a gated community by a neighbourhood watch patroller. “Any of those things will make him a target. I know the world. I’ve been in it,” concludes Dana, darkly.

But Gren is not willing to give up a world he’s never known. He is drawn to Herot Hall; drawn to Dylan, who finds in Gren an unlikely playmate; drawn to a terrible blood feud between his and Dylan’s families. Headley hits the beats of the original story while shifting the focus entirely. The Beowulf analogue here is police officer Ben Woolf, but he’s certainly not the hero: the roaming consciousness of the novel spends barely more time resting with him than it does, delightfully, with a pack of dogs (“We travel on perfume lines, drink them out of the air like you listen to the radio”).

Instead, either Dana or Willa could be the title character, depending on how you read “mere”. If it’s a geographical feature, then it’s Dana: her mountain camp is by the lake. If it means “only”, then perhaps it’s an ironic reference to Willa, who is a desperate and dangerous woman under the homemaker surface. She anticipates patriarchal requirements so perfectly that, as with Dana, it’s hard to tell whether she’s victim or accomplice. She plans dinner parties and pours cocktails, but she also sees that a balaclava looks like “a sucked dry head, skull and eyes missing, only skin left”. Even inside Herot, she knows that “You can’t fight nature … You can’t make it human. You can’t even make yourself human. Everyone is an animal.”

If everyone is an animal, why do some get classified as monsters? Both Dana and Willa recognise something fearful in Gren. For Willa, he’s the intruder who threatens her family; for Dana, he’s someone whose otherness makes him vulnerable. Both could be wrong, though. Headley, unlike the Beowulf poet, only shows us Gren as others see him, and neither Willa nor Dana has perfect vision. What Gren wants – a place by the hearth, the touch of a friend, love – makes him perfectly human. The question rippling through Headley’s ambitious novel is why we need to cast anyone as a monster at all.

• The Mere Wife is published by Scribe. To order a copy for £11.43 (RRP £12.99) go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99.